The major goal of this project is to explore further the relationship between tumor suppressor gene abnormalities (retinoblastoma, p53) and drug resistance of sarcomas. The investigators will use a modified tetracycline inducible vector system to express Rb, E2F and mutant p53 in sarcoma cell lines to study this relationship. In collaboration with Dr. Carlos Cordon- Cardo (Project II) the investigators will also measure the expression of certain cell cycle control genes (Rb, MDM-2, p 53, E2F and cyclin D) in sarcoma tumor samples and correlate this data with results of antifolate sensitivity (DHFR and TS inhibitors) as measured by the whole cell in situ thymidylate synthase assay. A major cause of inherent resistance to MTX in soft tissue sarcoma elucidated by previous studies supported by this grant is decreased retention, due to either defects in MTX uptake, or lack of retention due to low levels of polyglutamate formation. The investigators plan to build on this information and to test the concept that trimetrexate administered together with leucovorin will be non-toxic to the host, but will be cytotoxic to sarcoma cells with these attributes. The investigators will test tumors propagated in SCID mice, using clinically relevant schedules for these purpose. A third cause of decreased MTX retention will also be studied, the lysosomal enzyme gama-glutamyl hydrolase, that converts MTX polyglutamates to MTX. Further purification and cloning of this enzyme is planned, and the potentiating effects of inhibitors of this enzyme to potentiate MTX cytotoxicity determined.